There are many posts on "jealousy" "envy" if you do a tag search or a search. Have a look, perhaps it will help.
Take your time and ask her to go at your pace. The rush at the beginning of something new and wonderful is great for new lovers, but not so much for other partners. NRE is a difficult thing to navigate, but if everyone manages to there is a lot to be gained by poly. Being aware of that for the partner who gets to sit and watch is sometimes helpful. Maybe doing a tag search for "NRE" would be helpful also.
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